Methods for measuring temperatures of high temperature, electrically heated furnaces have long been known and include thermal, electric and optical technology. Commonly, thermocouple sensors are used for temperatures in the lower ranges while optical pyrometry is used for the higher ranges. Other known methods of temperature measurement include power transducers which measure power applied to the heater and then relate this data to temperatures for a particular condition, acoustic probes which relate speed of sound to temperature, and gas flow probes that relate the measurement of viscosity of the gas to temperature. However, the latter two methods, acoustic and viscosity, are not of significance in the field of high temperature furnaces and thus will not be discussed further.
In high temperature furnace applications, energy is transferred from the heating element to the work primarily by radiation. The transfer of heat relates to a function of temperature difference between the element and the work. It is also a function of surface emissivities and geometry. Heating element materials and shapes of heating elements determine the level at which they can operate. Temperatures in excess of certain levels can be detrimental or destructive to the element material. Measurement of heating element temperature through conventional means such as thermal, electrical, and optical requires the use expensive probes and instruments and does not allow the same instrument to be used for the measurement of the temperature of the work at the same time. However, for the optimum use of the equipment, a correct level of element temperature is of extreme importance.
Temperature measurement in high temperature furnaces involves two primary methods. One is the use of a thermocouple that can be used to measure the work, the plenum or the heating element or other parts of the furnace interior. The limitation of the use of a thermocouple is associated with the selection of the active element materials as well as insulation. Very seldom are thermocouples used for temperatures in excess of 2000.degree. Celsius. Often a mechanism for withdrawing a thermocouple and having an optical device take over the reading is used for applications where temperatures are greater than 2000.degree. Centigrade. Thermocouples used for measuring the heating element are subject to early failure because during rapid application of energy, heating elements will reach a higher temperature than at steady-state condition and heating elements also operate at a higher temperature than that which is specified for the work. For temperatures of certain type furnaces that operate to 3000.degree. Centigrade, it is impossible to measure the heating element temperature with a thermocouple. Further, if a thermocouple is used for measuring temperature of the heating element, it ties up one channel of instrumentation which otherwise could be used to measure temperature of the work. Therefore, a compromise is typically made by having the thermocouple read a furnace interior temperature or temperature in the uniform zone where it reads temperature which will be the temperature that the work eventually will reach under steady-state conditions, which is lower than the temperature of the heating element.
The other method of temperature measurement currently used is optical reading. The typical optical system consists of a pyrometer or an infrared thermometer which views the heat through a window and determines the temperature by infrared detection. This system is used primarily for higher temperature levels, and its accuracy is dependent upon certain factors such as emissivity settings and condition of the window. This instrument can be used to measure temperature on the heating element as well as any other objects in line of sight. Since the instrument requires the window which is usually located on a port in the vessel of the furnace and employs a rather expensive instrument, typical application often uses only one or seldom more than three optical reading devices. If a device is used to measure the heating element, as in the case with the thermocouple, it does not read the work temperature and vice versa.